Solutions for the deposition of protective surface layers on iron and zinc and process therefor



United States Patent 3,539,402 SOLUTIONS FOR THE DEPOSITION OF PROTEC- TIVE SURFACE LAYERS ON IRON AND ZINC AND PROCESS THEREFOR Christian Ries, Cologne-Kalk, Germany, assignor to Gerhard Collardin GmbH, Cologne-Ehrenfeld, Germany No Drawing. Filed Nov. 20, 1967, Ser. No. 684,491 Claims priority, application Germany, Feb. 16, 1967, C 41,515 Int. Cl. C23f 7/00 U.S. Cl. 148-614 12 Claims ABSTRACT OF THE DISCLOSURE Solution for the deposition of protective layers which can be used on iron, steel and zinc surfaces without changing the solutions. The latter consist of small quantities of complex fluorides of iron, titanium, zirconium or silicon and small amounts of nitrate ions or similar oxidizer in water. The pH must carefully be maintained at 1.8 to 3.5. The solutions act on the metal surfaces very rapidly, i.e., within 3 to 45 seconds, and are applied at temperatures between 5 and 95 C., preferably at room temperature. Applicataion to the surfaces can be in a bath, by a spray or by roller coating. A passivation post-treatment may be applied in the form of aqueous dilute chromic and/ or phosphoric acid or acid salts thereof.

The invention relates to the deposition of protective coatings on iron, preferably steel, zinc and zinc-clad surfaces, particularly on continuous sheet or coils, by means of acid solutions, whereby the same baths can even simultaneously be used under like conditions for all of these metals. A process for this deposition also is described.

The application of protective layers or coatings on steel, zinc or galvanized surfaces by means of acid solutions is known for the purpose of improving the corrosion resistance and the adherence of organic coatings subsequently applied, such as paints, lacquers or plastics. Phosphatizing solutions hitherto have been employed on steel surfaces, and phosphatizing or chromatizing solutions on zinc. These procedures are not fully satisfactory because the phosphate layers, while providing good protection against corrosion, inhibit the adhesion of the organic coatings, which makes itself felt particularly when subsequent shaping or deformation of the metal is to be carried out. Even the comparatively thin layers produced by phosphatizing processes which do not form continuous surface coatings show these drawbacks. The effect named is not as pronounced with chromatized zinc; however, chromatization has the disadvantage that the large quantities of eflluents require decontamination so that relatively high expenses are incurred. In addition, the adherence of organic coatings still leaves room for improvement even upon chromatization. This is particularly true for steel surfaces which had been hotor pot-galvanized.

The problem frequently presents itself, especially in continuous processes, of treating iron, steel and zinc or galvanized material in the same baths. It has been found necessary in practically all chemical surface protection processes, even in those which in principle are suited for the treatment of different metals, to adjust the conditions to the metal treated in order to obtain optimal results. These conditions are the bath compositions, temperatures, pH, and others. This is aggravated when hot dip galvanized materials are present which after galvanization undergo a smoothing treatment between rollers, a process known in Germany as Sendzimierverzinkung.

It is an object of the invention to provide a simple process for the deposition of protective layers on iron,

3,539,402 Patented Nov. 10, 1970 steel, zinc and zinc-clad (galvanized) surfaces which permits treatment of all these materials in like baths and under like operational conditions and which attains surfaces which are most satisfactory in regard to protection against corrosion, adherence of organic coatings and deformability after application of such coatings without their detachment.

Another object of the invention is to supply suitable solutions to carry out this process.

These objects are attained by treating these metal surfaces with an aqueous solution free of chromic, phosphoric and oxalic acids, which contains complex fluorides of iron, titanium, zirconium or silicon in amounts of 0.1 to 5 g./l. (grams per liter), calculated on the total cations, and further 0.3 to 10 g./l. nitrate ions or a corresponding amount of other oxidizers. The solutions have a pH of 1.8 to 3.5. The temperatures are between 5 and C., preferably ambient (room) temperature, i.e., between 10 and 50 C.; and the treatment is carried out within 3 to 45 seconds, and preferably 5 to 30 seconds.

The values named are the effective amounts, pH, temperatures and times.

It has been found that particularly suited are solutions of complex iron fluorides. These produce especially hard and lasting layers which provide good resistance to corrosion and excellent adherence of organic coatings even when the coated metal subsequently is deformed.

The cations are added to the aqueous bath solutions in the form of their soluble salts whereby, in the case of titanium, zirconium and silicon, their fluoride complexes are most applicable. However, it is feasible to form the complexes within the baths by incorporating therein soluble salts of these metals and corresponding quantities of fluoride ions. For instance, titanyl sulfate or zirconyl chloride can be employed. Iron can be added either as complex fluoride or, e.g., as sulfate, nitrate or chloride. The baths may contain mixtures of these metals or mixtures of several metal salts, When not entered in the form of complexes, the fluoride ions are incorporated in the baths as hydrofluoric acid or its neutral or acid soluble salts. A slight excess of fluoride ions over the stoichiometrical amount needed for complex formation does not have any adverse effect.

The bath solutions according to the invention preferably contain soluble nitrates as oxidizer. This permits operation of the baths at particularly low temperatures and short time cycles, thus resulting in very easy procedures. In the simplest instance, the process can be carried out using a solution of iron-III-nitrate and hydrofluoric acid.

However, other oxidizing agents can be used in lieu of nitrates. Primarily those agents are employed which are known as oxidizing accelerators for phosphatizing solutions, such as sodiumand potassium nitrates, chlorates, bromates, hydrogen peroxide and its adducts, e.g., melamine perhydrate or urea perhydrate (percarbamide), aliphatic and aromatic nitroand nitrosocompounds, e.g., nitroguanidine, picric acid, nitrophenols, monoand dinitrobenzenesulfonic acids. Mixtures of several of these materials may be employed.

The treating baths are to be practically free from chromic acid and chromium-VI-compounds, of oxalic acid, oxalates, phosphoric acid and phosphates. These compounds or their anions, respectively, inhibit the formation of the layers according to the invention because they themselves are layer-forming anions. Even comparatively small quantities, e.g., of phosphates, decrease the adhesion of organic coatings. Chromic acid and chromium- VI-compounds, moreover, are poisonous, and the effiuents must be decontaminated.

The pH of the solutions must be adjusted to 1.83.5, and preferably is between 2 and 3. Adjustment can be carried out with alkali solutions or acids other than named in the preceding paragraph. Only when these pH values and the concentrations previously named are maintained, equally good surface layers are produced on iron, steel, zinc and galvanized metal.

The treatment time is exceedingly short and usually lies between 3 and 45 seconds, preferably to 30 seconds. This time is short enough to carry the conveyors in continuous plants at high speed without the need for particularly large baths or spray zones.

The temperature of the baths is kept between 5 and 95 C. Preferred temperatures are ambient temperatures, i.e., substantially room temperature, between substantially and 50 C. This constitutes a further advantage of the process according to the invention because the baths do not require heating nor necessarily the maintenance of a given temperature.

The solutions can be applied to the metals by immersion, coating, or spraying. Coating rolls can successfully be employed. The metal surfaces are cleaned and degreased prior to the treatment, using the customary organic solvents or alkaline, acid or neutral cleansers. The pretreatment is governed by the degree of soiling of the materials.

In many instances, a passivation of the layers produced by the process according to the invention is desirable. This can be accomplished by using dilute aqueous phosphoric and/ or chromic acid, or their sodium-, potassiumor ammonium salts, respectively. The concentrations generally are between 0.01 and 5 g./l.

In a preferred embodiment, the protective layers produced are passivated with aqueous dilute chromic acid which contains chromium-III-ions. The effective concentrations generally are 0.2-2 g./l. CrO and 0.05-1 g./l. Cr O The layers then are dried after passivation. Prior to the passivation, a water rinse is opportune but not a strict requirement, especially when squeeze rolls are used.

The baths can be prepared by dissolving the required compounds in the water or by dilution of corresponding concentrates. These baths can be employed for long periods of time and can be replenished with the starting components to the constant point. Points of the solution are understood to be the amounts of n/ 10 aqueous NaOH in milliliters (ml.) which are consumed by 10 ml. bath solution to elfect a color change of bromophenol blue (free acid) or phenolphthalein (total acid). When baths are replenished with the same liquid or solid concentrates, an adjustment of the pH may become necessary after several such additions.

For this reason, the baths preferably are renewed with solutions of concentrates, respectively, which contain complex fluorides of iron, titanium, zirconium and/ or silicon and at least one oxidizer and have a ratio of free acid to total acid of 1:1.05 to 1.75.

The process according to the invention produces uni- .formly thin and readily deformable coatings on iron, steel zinc and galvanized surfaces which impart good corrosion resistance and excellent adhesion of organic coatings subsequently applied. The coating layers produced on steel are taupe to bluish-iridescent, those on zinc are gray and, slightly yellowish-iridescent. The process is easy to carry out, and the baths have a long working life.

Like baths can be used for all metals named at like operational conditions. This is a particular advantage in conveyor devices wherein iron-, steeland zinc-clad sheets and coils are continuously treated. This is so because in conventional devices the operational conditions are contingent upon the particular metal treated, so that the baths must be changed when changing from one kind of metal to another. It therefore is possible to run different metals through the baths without stoppage or changeover. It even is feasible to treat different metal coils or sheets simultaneously, e.g., side by side, if the bath, spray Zone or coating machine is wide enough.

The invention now will be more fully explained by the following examples. However, it should 'be understood that these are given merely by way of illustration, and not of limitation, and that it is intended to cover all modifications and embodiments which do not constitute a departure from the spirit and the scope of the invention as hereinafter claimed.

In the examples, temperatures are degrees centigrade. Percentages are weight percent. Solutions are aqueous.

EXAMPLE 1 A continuous device for coiled or long-strip bands was used, wherein steel and galvanized bands were simultaneously treated by an automatic spray at a dwelling time of 10 seconds in all treatment zones. The surfaces first were degreased with an aqueous alkaline solution of customary composition, and rinsed with cold water, followed by an exposure to an aqueous solution containing Fer, 1.25 NO3 1.5

and having a pH of 2.2, at room temperature. A second cold water rinse was then applied (room temperature), after which the surfaces were rinsed with an aqueous solution of 0.06% chromium-VI-ions and 0.02% chromium- IlI-ions, then dried.

Layers thus formed on the steel and zinc surfaces were uniform, thin and well deformable. These layers promoted excellent adhesion of organic coatings, e.g., paints, lacquers and plastic coatings. The color of the steel band was taupe to weakly bluish-iridiscent; that of the galvanized bands gray to weakly yellowish-iridescent.

After coating with any one of the organic compounds named, bending tests and Ericssen ductility tests with star pattern showed excellent results, i.e., the coatings did not peel off.

The spray solution was renewed to maintain it at con stant point using an aqueous concentrate containing Percent Fe(No -9 H O 4 HF (40%) 3 HNO;; (60%) 3 The protective layer formed did not change from the original even after frequent renewals and much use of the solution.

EXAMPLE 2 A similar device was used as in Example 1 with a dwelling time of 12 seconds. Iron-, steel and hot dip galvanized (sendzimierverzinkte) iron bands were degreased, rinsed and then simultaneously sprayed with a solution containing G./l. FeF 1.7 N0 3.5

and having a pH of 2.4, at room temperature, rinsed with water at ambient temperature, passivated and dried. Iron and steel bands were passivated with 0.1% NaH PO, solution; the galvanized bands with 0.02% chromic acid solution.

Passivation is an optional step and in many instances is not required.

The spray was the same for all materials which attained thin, uniform and well deformable protective layers. These exhibited excellent adhesion to organic coatings as named in Example 1. When changing over from one band material to another, no changes were required in treatment conditions, i.e., time, pH or temperature.

EXAMPLE 3 In lieu of the spray solutions named in Example 2, the following solutions yielded equally good results:

1 g./l. FeF 2 g./l. HNO pH 2.0 (adjusted with NaOH) Time: 10" Temp. 30

5 g./l. FeF 7.5 g./l. HNO pH 3.0 (adjusted with NaOH) Time: 5l0" Temp. 21

KgZl'Fs 2.5 g./l. HNO pH 2.5 (adjusted with NaOH) Time: Temp. 22

Equally good results were obtained when the materials traversed a bath or were coated.

While iron and zirconium compounds had been named in the Examples, like results were obtained with corresponding titanium and silicon compounds or mixtures thereof, and oxidizers other than nitric acid, i.e., all those named previously were successfully employed.

1 claim as my invention:

1. A continuous process for the simultaneous deposition of protective surface layers on surfaces of longstn'p bands of like and different metals selected from the group consisting of iron, steel, zinc and zinc-clad iron and steel, which consists essentially of continuously passing said bands through a contact zone and continuously contacting said surfaces of said bands with an aqueous solution which is free from phosphoric, chromic and oxalic acids, and which contains complex fluorides of cations selected from the group consisting of iron, and zirconium, in amounts of 0.1 to 5 g./l., calculated on the total cations present, plus 0.1 to 10 g./l. of an oxidizer selected from the group consisting of nitrate ions, sodium and potassium chlorates, sodium and potassium bromates, hydrogen peroxide, melamine perhydrate, urea perhydrate, nitroguanidine, picric acid, nitrophenols and monoand dinitrobenzenesulfonic acid; said aqueous solution having a pH of 1.8 to 3.5 and a temperature of substantially 5 to C.; said bands being continuously contacted with said aqueous solution over a period of time of 3 to 45 seconds; and continuously removing said bands from said contact zone.

2. The process as defined in claim 1, wherein said complex fluoride is that of iron.

3. The process as defined in claim 1, wherein said complexes are formed in situ by introducing soluble salts of said cations and a corresponding amount of hydrofluoric acid.

4. The process as defined in claim 3, wherein said salt is iron-III-nitrate.

5. The process as defined in claim 1, wherein said solutions are replenished by adding thereto concentrates containing like ingredients as the original solution and having a ratio of free acid to total acid of 1: 1.05 to 1.75.

6. The process as defined in claim 1, wherein said contacting is accomplished by dipping.

7. The process as defined in claim 1, wherein said contacting is accomplished by spraying the surfaces.

8. The process as defined in claim 1, wherein said exposure is accomplished by means of coating rolls.

9. The process as defined in claim 1, wherein, after completed contacting step, a passivation step is carried out by a second contacting step to an aqueous solution of a substance selected from the group consisting of phosphoric acid, chromic acid, sodium-, potassiumand ammonium salts of said acids, or mixtures thereof.

10. The process as defined in claim 9, wherein passivation immediately follows deposition.

11. The process as defined in claim 9, wherein between deposition and passivation a water rinse is applied to said surfaces.

12. The process as defined in claim 9, wherein said passivation is carried out with an aqueous chromic acid solution of 0.2-2 g./l. CrO and 005-1 g./l. Cr O References Cited UNITED STATES PATENTS 2,276,353 3/1942 Thompson 1486.2 2,357,219 8/1944 Mott 148-614 3,160,506 12/1964 OConnor et al. 148-6.2 X

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 1486.2 

